Many useful devices can be formed from thin epitaxial layers of semiconducting material grown on a substrate. For example, infrared detectors can be made from epitaxial layers of mercury cadmium telluride (HgCdTe) formed on a cadmium telluride (CdTe) or cadmium zinc telluride (CdZnTe) substrate. The electrical properties of an epitaxial layer depend in part upon whether the layer is formed as n-type material or p-type material. Because different types of devices require different electrical properties, the ability to create n-type or p-type materials in a controlled manner is very important.
N-type and p-type material may be formed by doping an epitaxial layer during either the growth of the layer or after growth. For example, dopants or impurities may be introduced into an epitaxial layer as the layer is grown by techniques such as liquid-phase-epitaxy (LPE), molecular beam epitaxy (MBE), vapor phase epitaxy (VPE), which include hot-wall epitaxy (HWE), chemical vapor deposition (CVD), metal-organic CVD (MOCVD), metal-organic vapor phase epitaxy (MOVPE), and organo-metallic vapor phase epitaxy (OMVPE). After the growth of the epitaxial layer, dopants may be introduced into the layer by techniques such as ion implantation and diffusion, or by the use of a diffusion source. The dopants for the epitaxial layer can be chosen to create either n-type or p-type layers, or distinct regions of n-type or p-type material within a layer.
In prior methods, an epitaxial layer was doped using either relatively slow diffusing dopants or intrinsic defects native to the epitaxial layer, such as vacancies created by post-growth annealing processes. Although it was recognized that rapidly diffusing dopants or impurities could be used to extrinsically dope the epitaxial layer, rapidly diffusing impurities were avoided for the following reasons. Because rapidly diffusing impurities tended to migrate to the substrate, the surface of the epitaxial layer, or the interface between the epitaxial layer and the substrate, these impurities would not uniformly disperse throughout the layer. Furthermore, an electrical p-n junction could be created between epitaxial layers by doping one layer with an n-type dopant and doping another layer with a p-type dopant. If a rapidly diffusing n-type or p-type dopant was used, however, such dopant would migrate between the layers. In other words, it was difficult to localize and uniformly distribute a rapidly diffusing impurity in a specific layer.
Accordingly, a need has arisen for a method for the controlled doping of epitaxial layers and related materials to create uniform n-type and p-type material.